A heater is used in an electronic apparatus such as an Office Automation (OA) apparatus, home electric appliances, and precision manufacturing equipment. The heater is used in a fixing device for fixing toner to a sheet in a fixing device such as a copying machine and a facsimile. In addition, the heater is used for erasing a print and the like in a rewritable card reader. The heater is configured by forming power supply electrodes, a conductor, and a resistance heating element on a substrate, and the resistance heating element generates heat by electric power supplied from the power supply electrodes.
In general, the heater used in the fixing device has silver and palladium, or ruthenium oxide, and glass as main components, and the resistance heating element having a Positive Temperature Coefficient (PTC) characteristic of which a resistance temperature coefficient [ppm/° C.] is 0 or positive is used.
The heater has an effective length that matches a maximum size (length of paper in a direction parallel to a longitudinal direction of the heater) of a recording medium (paper) which can be heated by the fixing device, that is, the effective length is equal to or greater than the maximum size. Therefore, when heating the recording medium that is smaller than the maximum size, in the heater having the PTC characteristic, a temperature of a region of a paper non-passing portion rises in a longitudinal direction of the heater. Then, if priority is given to suppressing the temperature rise in the region of the paper non-passing portion, it can be considered that a resistance heating element having a Negative Temperature Coefficient (NTC) characteristic of which the resistance temperature coefficient [ppm/° C.] is negative is used in the heater.
A heater 1D of related art is described with reference to FIG. 5. If resistance heating elements 5-1 and 5-2 of the heater 1D are formed of a material having the PTC characteristic, when paper having a size smaller than a size that is able to be carried continuously passes through the heater 1D, paper is not deprived of heat in the paper non-passing portion. Therefore, the temperature of the paper non-passing portion rises. Particularly, when thick paper having a small size passes through the heater 1D in large quantities, in order to raise the temperature that is lowered by the paper passing portion, large electric power is supplied on the heater 1D. As a result, the temperature of the paper non-passing portion excessively rises and there is a concern that a component such as a heating roller is led to deterioration and damage. In order to suppress the temperature rise of the paper non-passing portion, it is conceivable to form the resistance heating elements 5-1 and 5-2 with a material having the NTC characteristic. In the resistance heating element having the NTC characteristic, a resistance value is lowered as the temperature rises. Therefore, a heating amount of the resistance heating elements 5-1 and 5-2, which are the paper non-passing portion in end regions in a longitudinal direction of a substrate 2, is lowered and it is possible to suppress the temperature rise in the paper non-passing portion.
However, if the NTC characteristic of the resistance heating elements 5-1 and 5-2 is large, for example, when thermal runaway of the heater 1D is generated due to failure of a thermistor which performs temperature control and the like, the heating amount is excessively increased. Since the resistance heating elements 5-1 and 5-2 are not formed in both ends of the substrate 2, the temperature rise is small. As a result, a temperature difference in the longitudinal direction of the substrate 2 is great and excessive heat stress is generated. Since the heat stress exceeds breaking strength of the heater 1D in a short period of time, a phenomenon in which cracking occurs in the end portions of the heater 1D is generated.
If the resistance heating element of which the NTC characteristic is large is used as a heating element used in the heater, when the temperature of the paper non-passing portion of the heater excessively rises, the resistance value is lowered and then the temperature rise of the paper non-passing portion is suppressed. However, if a component such as the thermistor, which performs the temperature control of the heater, is failed, thermal runaway of the heater is generated, the heating amount is excessively increased, and thereby a phenomenon in which cracking occurs in the heater in a short period of time is likely to be generated.
Therefore, an object of an exemplary embodiment is to provide a heater, in which temperature rise of a paper non-passing portion of the heater is suppressed and damage of the heater is suppressed during thermal runaway, and a fixing device.